1. Field of the Invention
The present invention relates to a wireless base station and a communication method therefor. More specifically, the invention relates to a wireless base station separated into a center unit and a remote unit and to a communication method for communication between the center unit and the remote unit.
2. Description of Related Art
In the field of wireless communication systems, while changing is demanded to the fourth generation, the data rate has been enhanced, and the communication area has become ubiquitous, such that a need is considered to increase for system configurations of the type including a large number of small base stations with relatively low transmission power. In addition, in the case of a wireless base station, the system-installable site is restricted by the size of the system, such that technology called “radio on fiber (ROF)” is regarded important.
The ROF divides a base station into a center unit and a remote unit, and connects the units with an optical fiber communication path. According to the ROF, a remote unit device, forwhich the installation site is important, can be made compact by simplifying the function of the remote unit that is directly connected to an antenna, such that the impact of the installation site problem regarding the base station can be relaxed. For reduction of the circuit size of such a remote unit, an apparatus configuration is known in which, for example, the entirety of a base band section and a part of a radio frequency (RF) section are integrated into the center unit side, and the remote unit is provided only with an optical-electrical (O/E) converter unit, an electrical-optical (E/O) converter unit, and a power amplifier unit.
As another technical trend in the filed of wireless communication systems, attention is drawn to an array antenna called a smart antenna including multiple antenna elements. In the array antenna, weight calculation is performed on signals being transmitted and received through the multiple antenna elements, whereby transmission and reception directions of radio signals in the base station can be restricted. In addition, it is known that the array gain can be obtained from the weight calculation, or unnecessary interference signals can be reduced by the weight calculation.
As an example configuration with a combination of the ROF and the smart antenna, Japanese Unexamined Patent Publication No. 2001-94332 discloses a technique in which multiple RF signals corresponding to antenna elements of an array antenna are time multiplexed, thereby to synthesize the plurality of RF signals into a single signal. According to the technique disclosed in above Patent document, an object is to solve the problem of signal delay deviations (transmission time differences) occurring in an optical fiber in the event of parallel transmission of multiple signals by employing a wavelength multiplexing technique(“wavelength multiplexing,” hereafter).
In the case of parallel transmission of multiple signals in accordance with the wavelength multiplexing technique, since the signal propagation path in the optical fiber is different depending on the wavelength, a slight deviation occurs in the signal propagation time. When the center unit outputs transmission signals for the respective antenna elements to the optical fiber as RF signals (radio frequency signals), even a slight deviation of the signal propagation time causes a significant phase rotation. Therefore, even when transmission signals weighted by antenna element, a problem still occurs in that desired be an patterns cannot be formed as it is influenced by the phase rotation. According to the technique disclosed in the above Patent document, it is devised such that the multiple signals being supplied to the array antenna are time multiplexed so as to be output in the form of a single signal to the optical fiber, thereby to theoretically prevent the deviation.
On the other hand, Japanese Unexamined Patent Publication No. 2001-267990 discloses a technique as briefed hereinbelow. In the event of synchronous transmission of multiple signals to be supplied to an array antenna, the remote unit measures the signal propagation time deviation depending on the wavelength by using probe signals inserted by a center unit into the respective transmission signals, and then supplies the measurement results to the center unit, thereby to compensate for the deviation.
These conventional, previously proposed techniques will be described in more detail herebelow with reference to FIG. 8. A center unit 5 comprises a data generating unit 501 that generates transmission data, a signal processor unit 502, an RF unit 503, and a center-unit optical interface 504.
Data generated by the data generating unit 501 is input into the signal processor unit 502, and is converted into a plurality of signal streams (or, multiple signals) desired to be supplied the array antenna. The signal processor unit 502 executes three processings, namely, base band modulation, modulation in the spatial direction (array processing), and probe signal addition to the input signals.
For the base band modulation, coding with, for example, a convolution code or low density parity check (LDPC) code, anti-fading measures such as interleaving and repetition, and modulation such as quadrature phase shift keying (QPSK) and sixteen quadrature amplitude modulation (16 QAM) are executed. In the array processing, transmission weights are determined for the respective antenna elements, and weighting processing is executed on the respective transmission signal that has been base-band modulated. In the probe signal addition, a respective probe signal discriminatable from the transmission signal is generated and the probe signal is added to the transmission signal that has been array-processed.
Multiple transmission signals corresponding to the number of antenna elements being used for the transmission are output from the signal processor unit 502. In the state shown in FIG. 8, as an example, four streams of transmission signals are output from the signal processor unit 502. The transmission signals output from the signal processor unit 502 are input into the RF unit 503, and subjected to digital/analog conversion and frequency conversion thereon. The converted signals are then input into the center-unit optical interface 504. In the optical interface 504, the respective electric signals input from the RF unit 503 are converted into optical signals, and the optical signals are output in the form of a wavelength multiplexed optical signal to the optical fiber.
The optical signals being transmitted to the optical fiber are different in propagation characteristic depending on the wavelength. For this reason, deviations occur in, for example, phase and amplitude among the multiple transmission signals being supplied to the array antenna.
A remote unit 6 comprises a remote-unit optical interface 505, a power amplifier circuit 506, a signal detector unit 507, an array antenna 508, and a probe detector unit 509.
The remote-unit optical interface 505 executes processings in contrast with the center-unit optical interface 504 on multiple transmission signals input from the optical fiber, thereby converting a wavelength multiplexed optical signal into multiple analog RF signals. More specifically, a wavelength multiplexed optical signal input from the optical fiber is demultiplexed in terms of the wavelength by wavelength division processing, and optical signals of respective wave lengths are converted into electric signals by O/E conversion.
A plurality of signals output in parallel from the remote-unit optical interface 505 are amplified by the power amplifier circuit 506 comprising a plurality of amplifiers. In this event, deviations in the phases and amplitudes of amplified signals can occur depending on, for example, the differences in the characteristics of the respective amplifiers and temperatures. The transmission signals output in parallel from the power amplifier circuit 506 are input to the signal detector unit 507, and probe signals are detected from the respective transmission signals.
The transmission signals having passed through the signal detector unit 507 are transmitted as radio signals through the array antenna 508. The probe signals detected from the respective transmission signals by the signal detector unit 507 are input into the probe detector unit 509. The probe signals are transmitted together with the transmission signals through the optical fiber and the power amplifier unit 506 that causes deviations, and hence are input with the same deviations as those with the transmission signals into the probe detector unit 509. Accordingly, control parameters necessary for deviation compensation can be obtained by measuring the phases and amplitudes of the respective probe signals.
As shown by broken lines in FIG. 8, the control parameters obtained by the probe detector unit 509 are returned to the signal processor unit 502 of the center unit 5 through the remote-unit optical interface 505 and the center-unit optical interface 504. The signal processor unit 502 calculates compensation coefficients to be multiplied with the respective transmission signals and compensation amounts for delay times based on the respective control parameters, thereby performing signal processing to cancel the affects of the deviations on the transmission signals.
Generally, a wireless base station uses multiple frequency channels. Accordingly, if an array antenna is employed in the wireless base station, a considerable number of signal transmissions have to be performed between the center unit and the remote unit. This results in increasing in the number of optical fiber paths to be laid between the center unit and the remote unit, whereby reducing the advantage of low cost achieved by the division of the base station into the center unit and the remote unit.
According to any one of the conventional, previously proposed techniques described above, the system configuration is not such that portions of deviations occurring on the multiple transmission signals being transmitted from the center unit to the remote unit are restrictive so as to perform the deviation compensation within those portions in a closed form. More specifically, as shown by the broken lines in FIG. 8, the deviation compensating system is provided across both the center unit and the remote unit in the base station of the conventional configuration, so that means for passing the control parameters generated in the remote unit to the center unit has to be provided.